In order to understand the leukodystrophies, it is important to know some basic facts about the nervous system. The nervous system is made up of two main components: the central nervous system (CNS) and the peripheral nervous system (PNS). Together, these two systems interact to carry and receive signals that are responsible for nearly everything we do, including involuntary functions, such as our heartbeat, and voluntary functions, such as walking.
The CNS consists of the brain and the spinal cord and contains billions of specialized cells known as neurons. Neurons have projections called dendrites and axons that contribute to their unique function of transmitting signals throughout the body. Dendrites carry electrical signals to the neuron, while axons carry them away from the neuron.
The PNS consists of the rest of the neurons in the body. These include the sensory neurons, which detect any sensory stimuli and alert the CNS of their presence, and motor neurons, which connect the CNS to the muscles and carry out instructions from the CNS for movement.
Myelin, sometimes referred to as “white matter” because of its white, fatty appearance, protects and insulates the axons. It consists of a protective sheath of many different molecules that include both lipids (fatty molecules) and proteins. This protective sheath acts in a manner very similar to that of the protective insulation that surrounds an electric wire; that is, it is necessary for the rapid transmission of electrical signals between neurons. It does this primarily by containing the electrical molecules within the axon so that they are all properly transmitted to the next neuron. With the protective myelin coat, neurons can transmit signals at speeds up to 60 meters per second. When the coat is damaged, the maximum speed can decrease by ten-fold or more, since some of the signal is lost during transmission. This decrease in speed of signal transmission leads to significant disruption in the proper functioning of the nervous system.
The word leukodystrophy comes from the Greek words leuko (meaning white), trophy (meaning growth), and dys (meaning ill). Putting these pieces together, the word leukodystrophy describes a set of diseases that affect the growth or maintenance of the white matter (myelin).
All leukodystrophies are a result of problems with the growth or maintenance of the myelin sheath. There are many genes that are important in this process. For example, some genes are involved with the synthesis of the proteins needed for the myelin, while others are required for the proper transport of these proteins to their final location in the myelin sheath that covers the axons. Defects in any of the genes (called a mutation) may lead to a leukodystrophy; however, the symptoms of the individual leukodystrophies may vary because of the differences in their genetic causes.
Leukodystrophies are mostly inherited disorders, meaning that it is passed on from parent to child. One may be inherited in a recessive, dominant, or X-linked manner, depending on the type of leukodystrophy. The individual pages on each leukodystrophy will describe the particular pattern of inheritance for that disease.
There are some leukodystrophies that do not appear to be inherited, but rather arise spontaneously. They are still caused by a mutation in a particular gene, but it means that the mutation was not inherited. In this case, the birth of one child with the disease does not necessarily increase the likelihood of a sibling having the disease.
The leukodystrophies do share some common features with multiple sclerosis (MS). Like the leukodystrophies, MS is caused by the loss of myelin from the axons. However, the cause is different; whereas leukodystrophies are generally caused by a defect in one of the genes involved with the growth or maintenance of the myelin, MS is thought to be caused by an attack on the myelin by the body’s own immune system.
New leukodystrophies are always being identified. We try to keep our information as up-to-date as possible, and so the 52 leukodystrophies we have listed here comprise the defined leukodystrophies to the best of our knowledge.